OK, so I was happy coding in VS 2015 and adviced to upgrade to VS 2017. And so I did, and of course GrassHopper AS Template for RHino 5 refuse to install. WIP works fine though.
If your objective is to code for Rhino 5 in VS 2017, you could install the templates for RhinoWIP and just change the referenced assemblies to those used in rhino 5. I tested that last week and it worked fine. For debugging, don’t forget to change the application to Rhino 5.